Hydroxysteroid dehydrogenases (HSDs) regulate the occupancy and activation of steroid hormone receptors by converting steroid hormones into their inactive metabolites.
There exist numerous classes of HSDs. The 11 β-hydroxysteroid dehydrogenases catalyze the interconversion of active glucocorticoids (such as cortisol and corticosterone) into their inert forms (such as cortisone and 11-dehydrocorticosterone). The isoform 11-beta-hydroxysteroid dehydrogenase type 1 (11 β-HSD1) is expressed in liver, adipose tissue, brain, lung and other glucocorticoid tissue. 11 β-HSD1 is a potential target for therapy directed at numerous disorders that may be ameliorated by reduction of glucocorticoid action, such as diabetes, obesity and age-related cognitive dysfunction. Seckl, et al., Endocrinology, 2001, 142:1371-1376.
Glucocorticoids play a role in the development of diabetes. Glucocorticoids enable the effect of glucagon on the liver. Long et al., J. Exp. Med. 1936, 63: 465-490; and Houssay, Endocrinology 1942, 30: 884-892. In addition, it has been well substantiated that 11 β-HSD1 plays an important role in the regulation of local glucocorticoid effect and of glucose production in the liver. Jamieson et al., J. Endocrinol. 2000, 165:685-692.
Using 11 β-HSD1 inhibitors in the treatment of diabetes has been supported by various experiments conducted in mice and rats. These studies showed that the mRNA levels and activities of two key enzymes in hepatic glucose production, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase), were reduced upon administration of HSD inhibitors. In addition, blood glucose levels and hepatic glucose production were shown to be reduced in 11β-HSD1 knockout mice. Kotelevtsev et al., Proc. Natl. Acad. Sci. USA 1997, 94: 14924-14929.
HSDs also play a role in obesity. Obesity is an important factor in Syndrome X as well as type II (non-insulin dependent) diabetes. Omental fat appears to be of central importance in the development of both of these diseases, as abdominal obesity has been linked with glucose intolerance, hyperinsulinemia, hypertriglyceridemia, and other factors of Syndrome X (e.g., raised blood pressure, decreased levels of HDL and increased levels of VLDL). Montague et al., Diabetes 2000, 49:883-888, 2000. It has also been reported that inhibition of the 11β-HSD1s in pre-adipocytes (stromal cells) resulted in a decreased rate of differentiation into adipocytes. This is predicted to result in diminished expansion (possibly reduction) of the omental fat depot, which may lead to reduced central obesity. Bujalska et al., Lancet 1997, 349:1210-1213.
Inhibition of 11 β-HSD1s in mature adipocytes is expected to attenuate secretion of the plasminogen activator inhibitor 1 (PAI-1), which is an independent cardiovascular risk factor, as reported in Halleux et al., J. Clin. Endocrinol. Metab. 1999, 84:4097-4105. In addition, a correlation has been shown to exist between glucocorticoid activity and certain cardiovascular risk factors. This suggests that a reduction of the glucocorticoid effects would be beneficial in the treatment or prevention of certain cardiovascular diseases. Walker et al., Hypertension 1998, 31:891-895; and Fraser et al., Hypertension 1999, 33:1364-1368.
HSDs have also been implicated in the process of appetite control and therefore are believed to play an additional role in weight-related disorders. It is known that adrenalectomy attenuates the effect of fasting to increase both food intake and hypothalamic neuropeptide Y expression. This suggests that glucocorticoids play a role in promoting food intake and that inhibition of 11β-HSD1s in the brain may increase satiety, thus resulting in a decreased food intake. Woods et al., Science 1998, 280:1378-1383.
Another possible therapeutic effect associated with modulation of HSDs is that which is related to various pancreatic ailments. It is reported that inhibition of 11 β-HSD1s in murine pancreatic β-cells results in increased insulin secretion. Davani et al., J. Biol. Chem. 2000, 275:34841-34844. This follows from the discovery that glucocorticoids were previously found to be responsible for reduced pancreatic insulin release in vivo, Billaudel et al., Horm. Metab. Res. 1979, 11:555-560. Thus, it is suggested that inhibition of 11 β-HSD1 would yield other beneficial effects in the treatment of diabetes other than the predicted effects on the liver and fat reduction.
11 β-HSD1 also regulates glucocorticoid activity in the brain and thus contributes to neurotoxicity. Rajan et al., Neuroscience 1996, 16:65-70; and Seckl et al., Neuroendocrinol. 2000, 18:49-99. Stress and/or glucocorticoids are known to influence cognitive function (de Quervain et al., Nature 1998, 394:787-790). These reports, in addition to the known effects of glucocorticoids in the brain, suggest that inhibiting HSDs in the brain may have a positive therapeutic effect against anxiety and related conditions. Tronche et al., Nature Genetics 1999, 23:99-103. 11β-HSD1 reactivates 11-DHC to corticosterone in hippocampal cells and can potentiate kinase neurotoxicity, resulting in age-related learning impairments. Therefore, selective inhibitors of 11 β-HSD1 are believed to protect against hippocampal function decline with age. Yau et al., Proc Natl. Acad. Sci. USA 2001, 98:4716-4721. Thus, it has been hypothesized that inhibition of 11β-HSD 1 in the human brain would protect against deleterious glucocorticoid-mediated effects on neuronal function, such as cognitive impairment, depression, and increased appetite.
HSDs are believed to play a role in immunomodulation based on the general perception that glucocorticoids suppress the immune system. There is known to be a dynamic interaction between the immune system and the HPA (hypothalamopituitary-adrenal) axis (Rook, Baillier's Clin. Endocrinol. Metab. 2000, 13: 576-581). Glucocorticoids help balance between cell-mediated responses and humoral responses. Increased glucocorticoid activity, which may be induced by stress, is associated with a humoral response and as such, the inhibition of 11 β-HSD1 may result in shifting the response towards a cell-based reaction. In certain disease states, such as tuberculosis, leprosy, and psoriasis, the immune reaction is typically biased towards a humoral response when a cell-based response might be more appropriate.
Recent reports suggest that the levels of glucocorticoid target receptors and of HSDs are connected with the risks of developing glaucoma. Stokes et al., Invest. Opthalmol. 2000, 41:1629-1638. Further, a connection between inhibition of 11β-HSD1 and a lowering of the intraocular pressure was reported. Walker et al., poster P3-698 at the Endocrine society meeting Jun. 12-15, 1999, San Diego. It was shown that administration of the nonspecific 11β-HSD1 inhibitor, carbenoxolone, resulted in the reduction of the intraocular pressure by 20% in normal patients. In the eye, 11 β-HSD1 is expressed exclusively in the basal cells of the corneal epithelium, the non-pigmented epithelialium of the cornea (the site of aqueous production), ciliary muscle, and the sphincter and dilator muscles of the iris. No HSDs have been found at the trabecular meshwork, which is the site of drainage. Therefore, 11 β-HSD1 is suggested to have a role in aqueous production.
Glucocorticoids also play an essential role in skeletal development and function but are detrimental to such development and function when present in excess. Glucocorticoid-induced bone loss is partially derived from suppression of osteoblast proliferation and collagen synthesis, as reported in Kim et al., J. Endocrinol. 1999, 162:371 379. It has been reported that the detrimental effects of glucocorticoids on bone nodule formation can be lessened by administration of carbenoxolone, which is a non-specific 11 β-HSD1 inhibitor. Bellows et al., Bone 1998, 23:119-125. Additional reports suggest that 11-HSD1 may be responsible for providing increased levels of active glucocorticoid in osteoclasts, and thus in augmenting bone resorption. Cooper et al., Bone 2000, 27:375-381. This data suggests that inhibition of 11 β-HSD1 may have beneficial effects against osteoporosis via one or more mechanisms which may act in parallel.
There remains a need for inhibitors of 11 β-HSD1 for the treatment of 11 β-HSD 1-mediated conditions.